The speed of modern ink jet printers is steadily increasing for economic reasons. Recording sheets particularly suitable for these printers need to absorb the inks very rapidly. Recording sheets particularly suitable for this purpose comprise nanoporous inorganic compounds, preferably oxides, such as aluminum oxides or silicon dioxide, or oxide/hydroxides, such as aluminum oxide/hydroxides. Such recording sheets are known as “nanoporous” recording sheets. Particularly preferred nanoporous recording sheets are, for economic reasons, those containing, fumed silicon dioxide having a positively charged surface as nanoporous inorganic oxide.
Nanoporous recording sheets absorb the inks very rapidly (in the micro-second range) by the action of the capillary forces of the nanoporous compounds. Polymer based recording sheets absorb the inks more slowly (in the millisecond range) by swelling of the polymer.
Images produced in this way need to have excellent storage stability even under adverse conditions. This can only be achieved by using a finely tuned system of inks (respectively the dyes contained therein) adapted to these nanoporous recording sheets.
Such recording sheets do not have all the properties required when printed with most of the inks available today. In particular, the storage stability of images printed onto these recording sheets has to be improved. These images are not stable when they are in contact with ambient air, which normally contains impurities such as ozone and/or nitrogen oxides. They are strongly altered or even destroyed in a short time when they are in contact with ambient air. This phenomenon, also known as missing stability against degradation by ozone, is described for example by R. A. Barcock and A. J. Lavery in “Ozone Degradation of Ink Jet Photo Quality Images”, Journal of Imaging Science and Technology 48, 153-159, (2004).
Cyan inks for ink jet printing generally comprise copper phthalocyanine dyes such as Direct Blue 199, Direct Blue 86 or Direct Blue 87. Other phthalocyanine dyes used in ink jet printing are described by M. Fryberg in “Dyes for Ink Jet Printing”, Review of Progress in Coloration 35, 1-30 (2005).
The phthalocyanine dyes described therein do not, however, meet all the necessary requirements, because images printed with these dyes onto nanoporous recording sheets do not have a sufficient stability against degradation by ozone.
The phthalocyanine dye of formula (I) is described in patent application JP 2003-213, 153 (example No. 134).

This copper phthalocyanine dye shows an improved stability against degradation by ozone in comparison to copper phthalocyanine dyes used before, but its hue is too greenish and the dye is not sufficiently brilliant.
Dyes used for the preparation of inks for ink jet printing need to have a good solubility in the essentially aqueous ink liquid, they have to penetrate into the recording sheet and should not show dye aggregation on the surface of the recording sheet (“bronzing”). They need to provide printed images having high optical density, good water fastness, good light stability and good storage stability even under adverse conditions. They need to be stable in the ink even when the ink is stored for a long time under adverse conditions.
Various types of ink compositions have been proposed. Typical inks comprise one or more dyes or pigments, water, organic cosolvents and other additives.
The inks have to satisfy the following criteria:    (1) The ink gives images of excellent quality on any type of recording sheet.    (2) The ink gives printed images exhibiting good water fastness.    (3) The ink gives printed images exhibiting good light stability.    (4) The ink gives printed images exhibiting excellent smudge behavior.    (5) The ink gives printed images exhibiting excellent storage stability under conditions of high temperature and humidity.    (6) The ink does not clog jetting nozzles of the ink jet printers even when these are kept uncapped while recording is suspended for long periods.    (7) The ink may be stored for long periods without deterioration of its quality.    (8) The values of the physical properties of the inks, such as viscosity, conductivity and surface tension are all within defined ranges well suited for the intended use.    (9) The ink has to be non-toxic, non-flammable and safe.
Compositions for cyan inks for ink jet printing comprising phthalocyanine dyes are described for example in patent applications EP 1,405,883, EP 1,473,335, EP 1,475,417 and US 2003/0,041,775.
There is therefore still a need for improved, brilliant phthalocyanine dyes giving images on nanoporous recording sheets with excellent stability against degradation by ozone and showing a suitable bluish, saturated hue.